The present invention relates generally to circuit board arrangements, and more particularly to via arrangements that are used on printed circuit boards for high-speed electrical transmission applications.
In the field of data communication, data transfer speeds have steadily increased over the years. This increase in speed has required the development of high-speed electronic components for use in the telecommunications field, such as Internet use and use in data transfer and storage applications. In order to obtain an increase in the speed at which electrical signals are transmitted, it is known to use differential signals.
Twisted pair wires are commonly used to transmit differential signals and are most commonly used in electrical cables. These signal cables have one or more twisted pairs of wires that are twisted together along the length of the cable, with each such twisted pair being encircled by an associated grounding shield. These twisted pairs typically receive complimentary signal voltages, i.e., one wire of the twisted pair will carry a +1.0 volt signal, while the other wire of the twisted pair will carry a −1.0 volt signal. The wire pairs are twisted together along the axis of the cable so that each of the wires extends in a helical path along the cable and the wires are spaced apart from each other the same distance along this helical path for the length of the cable.
As the signal cables are routed on a path to an electronic device, they may pass by or near other electronic devices that emit their own electric field. These devices have the potential to create electromagnetic interference in the transmission lines formed by the signal cables. However, the twisted pair construction of the cables minimizes or diminishes any induced electrical fields by maintaining the two wires in a desired orientation so that they will capacitively couple to each other and to an associated grounding shield or drain wire, and this construction thereby substantially prevents electromagnetic interference from occurring in the cable and affecting the transmission of data signals through the cable.
In order to maintain electrical performance integrity from such a transmission line to the circuitry of an associated electronic device, it is desirable to obtain a substantially constant impedance throughout the transmission line, from circuit to circuit and to avoid large discontinuities in the impedance of the transmission line. Large discontinuties in the impedance of the transmission line can lead to the generation of undesireable crosstalk between the signal paths of the transmission line or electrical “noise”. Both this type of noise and crosstalk adversely affect the integrity of electrically transmitted signals at high frequencies (or data transfer speeds). The “transmission line” between electronic devices not only includes cables and connectors that interconnect two devices together, but also includes the printed circuit boards of the devices.
The impedance of twisted pair transmission cables may be controlled because it is easy to maintain a specific geometry or physical arrangement of the signal conductors and the grounding shield, an impedance change will usually encountered in the area where a cable is mated to a connector, where the connector is mounted to a printed circuit board and where the connector is mounted to a circuit board. This last area is referred to in the art as the “launch” area“where signals are launched from the transmission lines on (or in) the circuit board into a connector mounted thereto. Likewise, the signals may be launched from the connector into the circuit board and this area is commonly also referred to as an “exit” area. These areas are the same but may have different terms depending on the orientation and direction of the signal path, either from the circuit board to the connector or from the connector to the circuit board. The present invention is directed to improved structures used in these circuit board launch or exit areas.
Circuit boards are made up of multiple layers of conductive and nonconductive material. Each layer may be considered as defining one of multiple planes of the circuit board. A nonconductive layer may be used as a base of the circuit board and a surface or surfaces thereof may be coated with a conductive material such as a copper foil or plating. Portions of this are removed to form conductive extents on the surfaces of the board which are typically referred to in the art as “traces”. These traces define circuit paths on the board base layer. A subsequent nonconductive layer is then applied onto the surfaces of the base layer and another conductive coating is applied to that layer and etched into a pattern. A third nonconductive layer is applied over this second conductive layer and the process is repeated until a multi-layer circuit board is formed. The different conductive layers are typically connected together by what are known in the art as “vias”. A via is a hole that is drilled through the circuit board and the inner surface of which is plated. This plating interconnects the various conductive layers. The traces on the circuit board may lead to a via location when it is desired to connect the traces to other traces. Similarly, the vias may also be used to receive through-hole mounting pins or other mounting pins of connectors.
Pairs of traces may be formed in a circuit board layer to carry a pair of differential signals and each pair will define a differential signal transmission line of the circuit board. Each circuit board layer or plane, may support one or more such differential signal transmission lines. It is important to control the impedance of these transmission lines to minimize crosstalk and electrical interference during operation of the devices without unduly complicating the circuit board design and the circuit layouts on the circuit board.
The present invention is therefore directed to a circuit board design, utilizing circuit board vias and exits of conductive traces from the vias that cooperatively define an electrical signal transmission line, to provide a high level of operational performance and which maintains the desired electrical characteristics, such as the impedance of the circuit board signal transmission lines.